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(CBS News) A new study that finds children of a gay or lesbian parent may be more likely to have social and emotional problems has sparked controversy on both sides of the same-sex marriage debate.

The study, from Mark Regnerus, an associate professor of sociology at The University of Texas at Austin, surveyed more than 15,000 Americans between the ages of 18 and 39, asking them questions about their upbringings. Its findings are published in the July issue of Social Science Research.

One survey question asked whether a parent had been in a same-sex relationship during a child's upbringing; Regnerus wanted to see whether there were differences between kids raised in a household by a parent in a same-sex relationship compared with those who were raised by biological parents who were married and heterosexual.

The survey results were measured by a set of 40 outcomes on social, emotional and relationship factors. Outcomes included whether a child had grown up to need public assistance like welfare, were more likely to have anxiety or depression, were more likely to be abused, or were more apt engage in unhealthier habits such as having more sexual partners, smoking or using drugs.

Regnerus' analysis identified 175 now-adult children who said they were raised by a lesbian mother, along with 73 who said their father was in a same-sex relationship. Focusing on the larger sample, the study found respondents whose mother had a same-sex relationship fared worse on 24 of the 40 tested outcomes, compared with children of an intact heterosexual couple.

Sixty-nine percent of children of lesbian mothers reported that their family received public assistance, such as welfare at some point, compared with 17 percent from intact biological families. About half of children of an intact biological family said they were employed full-time, compared with 26 percent of those born to a lesbian mother. Fourteen percent of kids of a lesbian mom spent time in foster care at some point, compared with 2 percent of the rest of the children studied. Overall, less than 2 percent of all respondents who said their mother had a same-sex relationship reported living with their mom and her partner for all 18 years of their childhood.

With children of dads in a same-sex relationship, there were 19 outcomes they performed worse on, Regnerus told HealthPop, so they didn't have as many negative outcomes as kids born to a mom without the mother, but more than those who grew up in a home with married heterosexual parents, he said.

"Most conclusions about same-sex parenting have been drawn from small, convenience samples, not larger, random ones," Regnerus said in a news release. "The results of that approach have often led family scholars to conclude that there are no differences between children raised in same-sex households and those raised in other types of families. But those earlier studies have inadvertently masked real diversity among gay and lesbian parenting experiences in America."

Several experts and advocacy groups have taken issue with the study's methodology, saying a comparison of children of a lesbian mother -- who herself may have divorced the child's biological father, or may not even identify as a lesbian since the survey only asked if a parent had ever been in a same-sex couple during their childhood -- is an unfair, flawed comparison.

"Whether same-sex parenting causes the observed differences cannot be determined from Regnerus' descriptive analysis," said Cynthia Osborne, associate professor at the Lyndon B. Johnson School of Public Affairs at The University of Texas at Austin. "Children of lesbian mothers might have lived in many different family structures, and it is impossible to isolate the effects of living with a lesbian mother from experiencing divorce, remarriage or living with a single parent. Or it is quite possible that the effect derives entirely from the stigma attached to such relationships and to the legal prohibitions that prevent same-sex couples from entering and maintaining 'normal relationships'."

In a joint statement from the Family Equality Council, the Human Rights Campaign (HRC), Freedom to Marry, and the Gay and Lesbian Alliance against Defamation (GLAAD), advocates called the study a "flawed, misleading, and scientifically unsound paper that seeks to disparage lesbian and gay parents."

"Because of the serious flaws, this so-called study doesn't match 30 years of scientific research that shows overwhelmingly that children raised by parents who are LGBT do equally as well as their counterparts raised by heterosexual parents," said Human Rights Campaign President Chad Griffin.

Gary Gates, a researcher at the Williams Institute, a sexual orientation policy think tank at the University of California, Los Angeles, told LiveScience that a more fair comparison would've been of children of heterosexual or same-sex couples who were raised in similar homes, with no divorces, separations or foster care.

"All he found is that family instability is bad for children and that's hardly groundbreaking or new," Gates, who was not involved in the research, told LiveScience.

Other critics have alluded to the study's funding from conservative groups the Witherspoon Institute and the Bradley Foundation, suggesting Regnerus had a right-wing agenda.

Regnerus defended his study to HealthPop, saying he set out to do a population-based study, which is considered the "gold standard" in his field. Other study's samples, he said, interviewed "convenient samples" of people researchers knew, friends, or groups that are linked together somehow, but he wanted a totally random sampling.

"People will say I'm irresponsible without weighing in with stronger data," he said. "This is the best quality data we've seen so far. If they don't like the results, I'm sorry."

Regnerus was upfront about the funding from conservative groups, and said he pledged to groups involved that he would report whatever the data found, regardless of which way it leaned. What's more, he says some of the criticisms are valid and plausible.

"There are some valid criticisms that are being made, such as the measurement decision on who should be called a lesbian mother in this study," Regnerus said. "People might say that's irresponsible to do this study without all these stable lesbian couples in the study," he said, adding the random sampling only found two out of the 175 children who said they lived in a home with both same-sex parents throughout all 18 years. "I would have been happy to compare them but they did not exist in large enough numbers."

Regnerus said it's entirely possible that instability in the household led to some of the reported negative outcomes in adult children of same-sex parents. He said children of heterosexual couples in an unstable home were also found to fare worse than those in a stable environment.

"People gay or straight should stick with their partners, he said. "I think the study provides evidence of that."

In a commentary in Slate, William Saletan writes, "What the study shows, then, is that kids from broken homes headed by gay people develop the same problems as kids from broken homes headed by straight people. But that finding isn't meaningless. It tells us something important: We need fewer broken homes among gays, just as we do among straights. We need to study Regnerus' sample and fix the mistakes we made 20 or 40 years ago."